In the field of data communications a modem is used to communicate information from one location to another. Digital Subscriber Line (DSL) technology now enables modems, or other devices such as frame relay data service units (DSU's), to communicate rapidly large amounts of data. This communication scheme generally adheres to a model, known as the Open Systems Interconnect (OSI) Seven-Layer model. This model specifies the parameters and conditions under which information is formatted and transferred over a given communications network. A general background of the OSI seven layer model follows.
In 1978, a framework of international standards for computer network architecture known as OSI (Open Systems Interconnect) was developed. The OSI reference model of network architecture consists of seven layers. From the lowest to the highest, the layers are: (1) the physical layer; (2) the datalink layer; (3) the network layer; (4) the transport layer; (5) the session layer; (6) the presentation layer; and (7) the application layer. Each layer uses the layer below it to provide a service to the layer above it. The lower layers are implemented by lower level protocols which define the electrical and physical standards, perform the byte ordering of the data, and govern the transmission and error detection and correction of the bit stream. The higher layers are implemented by higher level protocols which deal with, inter alia, data formatting, terminal-to-computer dialogue, character sets, and sequencing of messages.
Layer 1, the physical layer, controls the direct host-to-host communication between the hardware of the end users' data terminal equipment (e.g., a frame relay access device connected to a PC).
Layer 2, the datalink layer, generally fragments the data to prepare it to be sent on the physical layer, receives acknowledgment frames, performs error checking, and re-transmits frames which have been incorrectly received.
Layer 3, the network layer, generally controls the routing of packets of data from the sender to the receiver via the datalink layer, and it is used by the transport layer. An example of the network layer is Internet Protocol (IP) which is the network layer for the Transmission Control Protocol over Internet Protocol (TCP/IP) widely used on Ethernet networks. In contrast to the OSI seven-layer architecture, TCP/IP is a five-layer architecture which generally consists of the network layer and the transport layer protocols.
The transport layer (Layer 4) determines how the network layer should be used to provide a point-to-point, virtual, error-free connection so that the end point devices send and receive uncorrupted messages in the correct order. This layer establishes and dissolves connections between hosts. It is used by the session layer. TCP is an example of the transport layer.
Layer 5, the session layer, uses the transport layer and is used by the presentation layer. The session layer establishes a connection between processes on different hosts. It handles the creation of sessions between hosts as well as security issues.
Layer 6, the presentation layer, attempts to minimize the noticeability of differences between hosts and performs functions such as text compression and format and code conversion.
Layer 7, the application layer, is used by the presentation layer to provide the user with a localized representation of data which is independent of the format used on the network. The application layer is concerned with the user's view of the network and generally deals with resource allocation, network transparency and problem partitioning.
The communications networks that operate within the OSI seven layer model include a number of paths or links that are interconnected to route voice, video, and data (hereinafter collectively referred to as "data") traffic from one location of the network to another. At each location, an interconnect node couples a plurality of source nodes and destination nodes to the network. In some cases, the sources and destinations are incorporated in a private line network that may include a series of offices connected together by leased-lines with switching facilities and transmission equipment owned and operated by the carrier or service provider and leased to the user. This type of network is conventionally referred to as a circuit-switching network. Accordingly, a source node of one office at one location of the network may transmit data to a destination node of a second office located at another location of the network through their respective switching facilities.
At any given location, a large number of source nodes may desire to communicate through their respective switching facilities, or interconnect nodes, to destination nodes at various other locations of the network. The data traffic from the various source nodes is first multiplexed through the source switching facility, then demultiplexed at the destination switching facility and finally delivered to the proper destination node. A variety of techniques for efficiently multiplexing data from multiple source nodes onto a single circuit of the network are presently employed in private line networks. For instance, time division multiplexing (TDM) affords each source node full access to the allotted bandwidth of the circuit for a small amount of time. The circuit is divided into defined time segments, with each segment corresponding to a specific source node, to provide for the transfer of data from those source nodes, when called upon, through the network.
Other data communications systems, in contrast, have not been as successful employing multiplexing techniques to enhance network efficiency further. In particular, frame-relay networks offer far fewer alternatives than their circuit-switching network counterparts. Frame-relay networks are often referred to as packet-switching networks. Packet-switching networks, as opposed to circuit-switching networks, allow multiple users to share data network facilities and bandwidth rather than providing a specific amount of dedicated bandwidth to each user, as in TDM. Instead, packet switches divide bandwidth into connectionless, virtual circuits. As is known, virtual circuit bandwidth is consumed only when data are actually transmitted. Otherwise, the bandwidth is not used. In this way, packet-switching networks essentially mirror the operation of a statistical multiplexer (whereby multiple logical users share a single network access circuit). Frame relay operates within layer 2 (the data link layer) of the OSI model, and is an improvement over previous packet switching techniques, such as X.25, in that frame relay requires significantly less overhead.
Frame relay networks transport data packets by compressing and then transporting the packets over the communications channel. The communications channel can be provided by, for example, an ISDN (Integrated Services Digital Network) connection, however, other physical connections may be employed to transport frame relay data.
Frame relay is one of a series of data link protocols that fall within the broader class of data link protocols known as High-Level Data Link Control (HDLC). Typically, the frame relay data to be compressed and transported is packetized into standard High-Level Data Link Control--Unnumbered Information (HDLC-UI) Frame Relay packets in accordance with the Internet Engineering Task Force (IETF) Request for Comment (RFC) 1490. RFC 1490 defines the format of standard HDLC-UI frame relay packets. These standard packets are then compressed and transported over the frame relay network.
A frame relay network effectively comprises a mesh of interconnected nodes that can be variably configured to provide a communication path between a plurality of end points. A permanent virtual circuit is a configuration of various intermediate nodes in the frame relay service network which are dedicated to direct communications between a first end point and a second end point along a particular path.
Data link connection identifiers (DLCI's) are numbers that identify the various intermediate nodes between end points of a frame relay communication link. DLCI's identify the various intermediate nodes and therefore identify the particular communication path between end points. As is known, the provision of permanent virtual circuits simplifies the error control mechanisms for data transport utilizing frame relay technology, as opposed to communication links which are based on IP addresses and routers (where the communication path may vary from packet to packet).
Present frame relay switching devices process the flow of frame relay data and perform frame relay switching using a microprocessor under software control. The packets are sequentially processed by a microprocessor on data structures in a common memory. Data structures in microprocessor based implementations are accessed on some multiple of a byte regardless of the logic structure of the data. As a result, a software implementation cannot forward and switch an inbound packet until the entire packet has been received and is present in the microprocessor memory.
Accordingly, there is a clear need for an alternative implementation that provides for real time concurrent multiple processes, thereby reducing latency for symmetrical ports with DLCI switching. This is accomplished in the present invention by performing parallel processing of frame relay signals, which allows the frame relay access device to begin forwarding a packet as soon as the packet header is received (i.e., "cutting through" the packet) instead of waiting until the entire packet is present in the microprocessor memory, as is now done.